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This is a page from the manual for the Game Boy Color:

Manual page containing §4 ‘Installing batteries’, with the first paragraph stating: ‘Remove the cover on the back of the Game Boy Color and insert two AA batteries as shown below. For best results use two fresh, high quality alkaline batteries. Do not use rechargeable nickel-cadmium batteries.’

There is no explanation as to why this is, or how this can be, but it claims that one must not use rechargeable nickel-cadmium batteries.

Why is this? I don't know if any other kind of rechargeable batteries existed in 1998 when it was released, but I know that some form of rechargeable batteries existed. Are they really saying that any kind of rechargeable batteries cannot be used in the GBC? Or just specifically nickel-cadmium ones? Either way, why would it matter what kind of battery it is, as long as it has the same "interface" and size?

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    It's usually “interface”; different batteries have different voltages, because of the chemistry of the cells; you can only have integer multiples of the voltage that the chemicals produce. (I don't actually know why the Game Boy has this, though.)
    – wizzwizz4
    Jan 8 at 22:59
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    It's probably as simple as what wizz says. Alkaline batteries -- your basic, non-rechargeable AA -- will deliver around 1.5V when fresh. NiCad batteries start at 1.2V and just go down from there. Different devices will handle the lower voltage differently. Some will run just fine; some will continuously warn you the batteries are "low" when they aren't, and some will hardly run for long, or at all. Or, worse, your device "works" but gives weird results! Jan 8 at 23:02
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    I used rechargeable batteries in my GB and GBC for years without any adverse effects.
    – Valorum
    Jan 9 at 9:09
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    As I recall, I always used rechargeable batteries in my original Game Boy, back when it was brand new in 1989. The users guide for that one also advised against rechargeable batteries; I didn't realize until now that NiCad are only 1.2V - (whoops!) - but I never had a problem either, even though I played it constantly. Fun Fact: there were 1048 games for the Game Boy (excluding any cancelled and unlicensed games.)
    – ashleedawg
    Jan 9 at 10:01
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    Just to note, such a warning was very common on all sorts of devices. I remember a joke/conspiracy being that it was a ploy by Big Battery
    – eps
    Jan 9 at 18:23
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I suspect more "don't disappoint the users" than "harm to the device".

From the Wikipedia AA article, not only is the nominal voltage lower (1.2V vs. 1.5V for both alkaline and cheap zinc-carbon), the "Max. energy at nominal voltage and 50mA drain" is *significantly lower:

  • Alkaline = 3.90 Wh = 52 hours (3.9 Wh/50mA/1.5V = 52)
  • Zinc-carbon = 2.55 Wh = 34 hours
  • NiCd = 1.20 Wh = 20 hours

I don't know the actual power requirements of the Game Boy Color. According to Wikipedia, it used two AA batteries or a 3V/0.6W adapter and up to 10 hours of gameplay on batteries. 3.9Wh x 2 = 7.8Wh/10 hours = 0.78W, which is close to a nominal 0.6W on the adapter. That would translate into 6 hours on Zinc-carbon or 4 hours on NiCd.

The 3V adapter confirms the nominal target voltage of 3V. It really is quite plausible that the Game Boy Color would not work very well even with freshly charged NiCd due to the 2.4V nominal output. But even if it did, it wouldn't last long.

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    Zinc-carbon batteries spend at least half of their discharge below 1.2V. When a device is made to work off two zinc-carbon (or alkaline, for that matter) cells, it is pretty much made to work acceptably down to ~1.0V, otherwise the user will have to throw half-full batteries. NiCd on the other hand are quite stable at their 1.2 and abruptly fall below 1.0V when empty.
    – fraxinus
    Jan 9 at 9:00
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    @fraxinus And that's actually one of the side effect of NiCd in a gameboy; the system just dies without the telltale dimming of the power lights.
    – Nelson
    Jan 9 at 12:45
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    And don't forget that NiCad cells improved over the years, the 1000mAh assumed in the answer are about "modern" cells, 500mAh might be more realistic back then. Jan 10 at 11:13
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    @trognanders We're talking here about NiCd, not NiMh. Jan 10 at 18:45
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    I see. Actually, I did used rechargeable batteries in my GameBoy Color and I did notice that they didn't last as long as normal batteries. Jan 11 at 12:40
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Two simple reasons.

Different battery chemistries have different voltage and safety.

Standard alkaline batteries are nominally rated for 1.5V, but they can have more than 1.6V when brand new and are considered to be discharged at around 1.1V or so, depending on application.

Standard rechargeable batteries that are intended to be used in place of standard alkaline batteries in consumer electronics have less voltage. Nominally they are rated for 1.2V, for example NiCd batteries can have around 1.4V when fully charged and are considered to be discharged at around 1.0V.

So basically, the device is designed to use 1.5V alkaline batteries, and when using NiCd rechargeable batteries, the device can complain about batteries being empty and shut itself down, even though batteries are not empty yet. In case of NiCd batteries, they degrade faster when only partially charged and recharged, as they perform better when fully discharged before recharging.

And alkaline batteries are safer than rechargeable batteries, especially in the hands of children, as the device is basically a toy.

Rechargeable batteries are able to output roughly 10 times more current than alkaline batteries, so in the event of device malfunction due to mishandling such as dropping it, a short circuit with alkaline batteries is relatively harmless as the wires or batteries may heat up noticeably, but with rechargeable batteries, a short circuit can heat up copper wires red hot, melt the insulation, and start a fire. Not to mention possible hazards when mishandling the batteries outside the unit that might cause them to short, such as keeping spare rechargeable batteries carelessly.

The manual only talks about NiCd batteries, as while NiMH batteries did exist, they might not have been so popular everywhere due to various reasons.

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    NiMH batteries of the time were of the high self-discharge variety, and would go dead after just a month or two on the shelf. Low self-discharge NiMH batteries didn't appear until seven years after the GBC.
    – Mark
    Jan 10 at 22:14
  • "Rechargeable batteries have less voltage" isn't universally true, even if you're referring to individual cells, such as AA. For example, a Li-Ion 14550 cell (same dimensions as AA) is 3.7V. Jan 11 at 14:18
  • @TobySpeight: True, but the OP specifically asks about NiCd. And how popular were lithium batteries in 1998 anyway?
    – dan04
    Jan 11 at 18:02
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Most likely reason I think is that when NiCd batteries discharge, they stay at almost 1.2V for most of their discharge cycle then go from 1.2 to 0 volts very quickly, so the low batt warning is unlikely to be on for very long if at all, and the player being unable to save the game, if it simply turns itself off with little to no warning.

Since alkaline batteries reduce their voltage output as they discharge, 1.5V being the voltage only when unused, 1.2V is within the operation of almost any device that can use alkaline batteries (0.7-0.8V being a common low limit)

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